J. Phys. Chem. 1988, 92, 5387-5393 atoms and released an average of about 20 kcal/mol into translation. Essentially all the acetoxyl radicals underwent secondary decomposition to give CH3 and C02 with a surprisingly large release of translational energy. With an MPD rate equation model, the activation barrier for the concerted reaction of methyl acetate was determined to be 69 f 3 kcal/mol by assuming an endothermicity of 83.4 kcal/mol for simple bond rupture. All of the concerted reactions (1,2, and 8) where an H atom is transferred in a cyclic transition state released about 60% of the exit channel barrier into translational
5387
energy. This was interpreted in terms of a late transition state after which the closed-shell products, formed close to their equilibrium geometries, strongly repel each other. This work is being pursued further to explore the reaction dynamics of different types of transition states.
Acknowledgment. We thank Xinsheng Zhao for help with computer programming. This work was supported by the Office of Naval Research under Contract No. NOOO14-83-K-0069. Registry No. CH3COOC2H5, 141-78-6; CH3COOCH3, 79-20-9.
Photofragmentation of Acetone at 193 nm: Rotational- and Vibrational-State Distributions of the CO Fragment by Time-Resolved FTIR Emission Spectroscopy Eric L. Woodbridge, T. Rick Fletcher, and Stephen R. Leone*'+ Joint Institute for Laboratory Astrophysics, National Bureau of Standards and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 (Received: January 4, 1988)
Photofragmentation of acetone at 193 nm is known to produce predominantly two methyl radicals and a CO molecule. Infrared emission is collected from the CO fragment at time delays of 6-30 ps after the exciting laser pulse by a time-resolved Fourier transform spectrometer. High-resolution vibration-rotation spectra are obtained both under collision-free and rotationally relaxed conditions. The observed vibrational distribution is, for v = 11213, 0.73 k 0.04/0.23 A 0.02/0.04 & 0.01. This distribution fits a temperature of -20302$ K, in approximate agreement with previous low-resolution results from this laboratory. The observed rotational distribution is highly excited, approximating a temperature of 3360?j&lo"K. Both the vibrational and rotational distributions can be suocessfully modeled by using a "pure impulsive" mechanism for the fragmentation of an initial bent acetyl fragment. Thus, the observed distribution supports previous arguments that the dissociation occurs by a two-step fragmentation mechanism following single-photon excitation.
I. Introduction In recent years there has been a substantial amount of experimental and theoretical work on the dissociation dynamics of small polyatomic The utilization of sophisticated techniques in laser spectroscopy and mass analysis allows a detailed examination of state-testate cross sections, excited-state lifetimes, and the dynamical interactions of separating fragments. As Morse and Freed14 have pointed out, a detailed treatment of photodissociation has only been adequately described for diatomic molecules, where the limited degrees of freedom make the application of energy and angular momentum conservation laws straightforward. Additional degrees of freedom which are introduced in even the simplest triatomic molecule complicate matters greatly. A three-body process, such as the 193-nm fragmentation of a c e t ~ n e , ~presents ~ . ' ~ still greater challenges. There has been little consideration in the past of some of the most fundamental questions about this dissociation, such as the lifetime of the excited state or the time scales for breaking the two C-C bonds. R. S. Lewis and E. K. C. Lee reviewed in some detail the photoprocesses in simple aldehydes and ketones.2 These molecules generally exhibit strong absorptions in the visible and ultraviolet regions and undergo rearrangement or fragmentation upon excitation, producing many interesting radical species. Acetone has been the subject of many investigations, both experimental and theoreti~al.'~-~'At longer wavelengths, it has been established that the primary products of single-photon excitation are CH3C0 + CH3. Breuer and Lee19 and Hansen and LeeZ2 studied fluorescence lifetimes and rates for nonradiative processes of the first excited singlet state, revealing that intersystem crossing to the first excited triplet state is the dominant relaxation mechanism. At higher photon energies, the mechanism changes and both C-C bonds are broken. Yet the time scale for breaking of the two C-C Staff member, Quantum Physics Division, National Bureau of Standards.
0022-3654/88/2092-5387$01 SO10
bonds in the vacuum-UV excitation, Le., whether or not this is a concerted process, has never been definitively established.
(1) Gelbart, W. M. Annu. Reu. Phys. Chem. 1977,28,323.
(2) Lee, E.K.C.;Lewis, R. S. Ado. Photochem. 1980,12,1. (3) Shapiro, M.; Bersohn, R. Annu. Rev. Phys. Chem. 1982, 33,409. (4)Greene, C. H.;Zare, R. N. Annu. Rev. Phys. Chem. 1982,33,119. (5) Leone, S.R. Ado. Chem. Phys. 1982,50,255. ( 6 ) Simons, J. P. J. Phys. Chem. 1984,88,1287. (7) Bersohn, R.J . Phys. Chem. 1984,88,5145. (8) Brumer, P.; Shapiro, M. Adu. Chem. Phys. 1985,60,37. (9) Baht-Kurti, G. G.; Shapiro, M. Adu. Chem. Phys. 1985,60,403. (10)Jackson, W. M.; Okabe, H. Adu. Phorochem. 1986,13,1. (11) Kresin, V. Z.;Lester, Jr., W. A. Adu. Photochem. 1986, 13, 95. (12)Docker, M. P.; Hodgson, A.; Simons, J. P., to be published. (13) Houston, P. L. J . Phys. Chem. 1987,91,5388. (14)Morse, M.D.; Freed, K. F. J . Chem. Phys. 1981,74,4395. (15)Baba, M.; Shinohara, H.; Nishi, N.; Hirota, N. Chem. Phys. 1984, 83,22 1. (16)Donaldson, D. J.; Leone, S. R. J . Chem. Phys. 1986,85,817. (17)Potzinger, P.; Von Biinau, G . Ber. Bunsen-Ges. Phys. Chem. 1968, 72,195. (18)Soloman, J.; Jonah, C.; Chandra, P.; Bersohn, R. J . Chem. Phys. 1971,55,1908. (19)Breuer, G . M.; Lee, E. K. C. J . Phys. Chem. 1971,75,989. (20) Hancock, G.; Wilson, K. R. In Proceedings, Fourth International Symposium on Molecular Beams, Cannes, France, 1973, NTIS Microfiche, AD-771 320. (21)Huebner, R. H.; Celotta, R.J.; Mielczarek, S. R.; Kuyatt, C.E. J. Chem. Phys. 1973,59,5434. (22)Hansen, D. A.;Lee, E. K.C. J. Chem. Phys. 1975,62, 183. (23)Hess, B.; Bruna, P. J.; Buenker, R.J.; Peyerimhoff, S. D. Chem. Phys. 1976,18,267. (24)Trott, W. M.; Blais, N. C.; Walters, E. A. J . Chem. Phys. 1978,69, 3150. (25)Rogers, J. D.; Rub,B.; Goldman, S.; Person, W. B. J . Phys. Chem. 1981,85,3727. (26)Gaines, G . A.;Donaldson, D. J.; Strickler, S. J.; Vaida, V. J . Phys. Chem., in press.
0 1988 American Chemical Society
5388 The Journal of Physicul Chemistry, Vol. 92, No. 19. 1988
Woodbridge et al.
Potzinger and Von Bunau" photolyzed acetone at 185 nm with a low-pressure mercury arc and determined quantum yields (@) of the final products to he 1.0 (CO) and -0.95 (CzH,). generated by the mechanism CH,COCH, + hv CH, CH, CO (1)
-
-
+
+
followed by recombination of the methyl radicals to form CzH,. Pilling and co-workerszs later determined the accurate quantum yield for this pathway at 193 nm to be -0.96. Earlier, Baba et a1.I5 studied the multiphoton ionization (MPI) and fragmentation of acetone at 248 and 193 nm. The laser power dependence of the major ion product signals (CH,COCH,+ and CH,CO+) at 193 nm suggests that the dominant single-photon process in acetone is fragmentation to yield initially highly energized CH3C0 and CHI, with subsequent fragmentation of the acetyl radical to yield a second CH, and CO. Donaldson and Leone1, recently investigated the 193-nm fragmentation of acetone by observing the direct infrared emission from the energized CHI and C O fragments. Although the resolution of the experiment was limited ( - 3 M O c d ) , they were able to fit a temperature of -1200 K to the CO vibrations. The rotational contour of the vibrational emission, however, defied attempts to fit a single rotational temperature to the excited states. If indeed the first step in the single-photon fragmentation of acetone is to cleave one of the C-C bonds to yield CH, and CH,CO (with excitation in the C-C-0 bend), then the unimolecular fragmentation of CH,CO should give a substantial rotational "kick" to the departing CO. This might result in an excited distribution of J states whose character is determined largely by the kinematics of the dissociation of CH,CO. Acetone has a large absorption cross section at 193 nm, -3.8 X IO-'' cmz,15whichresults in -6% fragmentation of the parent molecules. Dissociation follows excitation to a l(n.3~)Rydberg state') and, as indicated, produces two methyl radicals and the ground-state CO fragment with a quantum efficiency of -0.96?8 Recent experimentsz9have determined the branching ratios for other minor fragmentation pathways, including (CH,),CO CHzCOCH, + H (52.5%) (2) (CH,),CO
--
CHzCO + CH, (51.5%)
(3) These same experiments suggest that secondary photolysis of CHI is also negligible. In this experiment we employ the new method of time-resolved Fourier transform infrared emission spectr~scopy."~~With this technique we can examine many molecular photoprocesses that are not amenable to laser-induced fluorescence, MPI, or other detection methods. The high-resolution capability inherent in interferometric instruments gives us the ability to observe directly and unambiguously the energy disposal in internal emitting states of photofragments, providing a detailed picture of the intra- and intermolecular dynamics that govern the photodissociation event. For the 193-nm photolysis of acetone, we confirm under high resolution that the vibrational distribution in the CO fragment is similar to the previous estimate', and extract information about the highly excited rotational distributions within each vibrational level of CO. 11. Experimental Section
The experimental apparatus has been described in detail elsewhere."' Figure 1 shows a diagram of the experimental arrangement. An ArF excimer laser beam is passed through a photolysis chamber through which the parent reagent also flows. (27) Donaldson, D. 1.;Gain-, 0. A,; Vaida, V.J , Phys. Chcm., in press. ( 2 8 ) Brouard. M.; MaePherson, M. T.; Piiling, M. I.; Tulioch, I. M.; Williamson, A. P. Chem. Phys. Lerr. 1985, i i 3 , 413. (29) Pilling, M. I., private communication. (30) Fletcher, T. R.; Leone, S. R. J . Chem. Phys. 1988,88,4720. (31) Donaldson. D.J.; Lcone, S. R. Chem. Phys. Lerr. 1986, 132. 240. (32) Excellent tnatmenb of FTIR theory and Michaelson interferometry may k found in the following: (a) &Ii, R. 1. Inlmductory Fourier Tmmfwm Spectroscopy; Academic: New York. 1972. (b) Griffiths, P. R. Fo'ourier Tramform InfwmdSpeermmetry; Wilcy-Intemienee: New York, 1986. (c) Chantry. G. W. Submiilimetre Speclroscopy; Academic: New York, 1971.
Figure 1. A perspective view of the experimental apparatus. A portion of the vacuum chamber has been removed f a clarity. See text for details. TIMING SEQUENCE
c -3mscc
4 REFERENCE INTERFERENCE FRINGES
I
/ I
I
TIMING PULSES M E DELAY
ZERO CROSSING I N I T I A T E S ONE DATA ACOUISITION SEOUENCE
Figure 2. Schematicdiagram of the timing sequence. See text for details.
The parent absorbs a single photon and undergas a unimolecular fragmentation. The infrared emission from the photofragments is collected into the FTIR, and the spcctmm is recorded. The high repetition rate excimer laser is synchronized to the mirror sweep of the FTIR to obtain time resolution. The laser beam is telescoped by two cylindrical lenses to obtain a beam profile 1 cin on each side. This beam passes into the photolysis chamber along ?p axis perpendicular to the optical axis of the interferometer. The beam is passed -12-15 times in the horizontal plane a 4 the $1 by dielectric minors that are -98% reflective at 193 nml 'With a laser pulse energy of -20 mJ we have enough fluence'to fragment -6% of the parent molecules in the volume of the $earn: Two-photon processes are negligible. The infrared emission from the photofragments is collected by two gold-coated spberical.mirrors, which are split in half, that define a multipass White cell collection system. The light output from the White cell'is turned by another gold-coated mirror and then focused into the emission port of the FTIR by a third spherical mirror. All of the IR optics are >98% reflective throughout the mid-IR region. Parent molecules are introduced through a stainless steel cylindrical manifold -1.9 cm in diameter and 10 cm in length. Twenty-five holes of 0.04-+ diameter are arranged in five rows along the bottom of the manifold. The chamber is pumped by a 5200 L s-l oil diffusion pump hacked by a 17 L s-' mechanical pump. For these experiments the diffusion pump was not used. The ultimate pressure utilizing only the mechanical pump is 50.5 Pa. The time resolution of the experiment is obtained as shown in Figure 2. A single-frequency He:Ne laser is passed through the two arms of the interferometer, generating an interference pattern (interferogram) as the moving mirror sweeps which consists of a single sine wave. The positive zero crossings of this sine wave
-
The Journal of Physical Chemistry, Vol. 92, No. 19, 1988 5389
Photofragmentation of Acetone at 193 nm
0 EXPT.
+
a
0 CALC., 2030 K 0.6
1
2
0.5
-i 5
I 0
0.2
-I
w 2000
2102
-
are used to mark the position of the moving mirror in the interferometer. These zero crossings are also utilized to initiate digitization of the infrared signal falling on the detector. Thus as the mirror sweeps, an interferogram is built up that consists of discretely sampled points taken at every positive zero crossing of the He:Ne laser fringes. This is the normal operating mode of many commercial Fourier transform spectrometers. The delay time between the positive zero crossing of a He:Ne laser fringe and the digitizing of the infrared signal, t l , is software selectable through the interferometer and can be varied from 0.1 to 1600 ps. Typically in this experiment we delay 160 pus before sampling the infrared signal. The laser is pulsed by taking the signal from the positive zero crossing of the He:Ne laser fringe, sending it to a delay generator, and delaying the signal by some time t2 before the laser is triggered. Thus by simply making t2 less than t l , we pulse the laser immediately before the signal on the detector is sampled. The time delay is then At = tl - t2. The detector is a 1.O-mm-diameter Ge:Hg element in a liquid helium Dewar with a single MOSFET as the first stage of amplification. The detector element and the MOSFET are operated at -4 K. The Dewar is equipped with a 4 K cooled filter wheel and a band-pass filter which restricts the wavelength region detected to 1800-2300 cm-'. The output of the detector is amplified by a high-gain, low-noise amplifier and then sent directly to the analog-to-digital converter of the interferometer. The raw spectra obtained are a convolution of the actual emission intensity with the response function of the detector, the transmission/reflection functions of all the optics, and the blackbody background. To correct the spectra, a very high signal-to-noise ratio spectrum is taken of the 300 K ambient radiation without pulsing the laser. This blackbody spectrum is then divided by a calculated 300 K blackbody spectrum to give an approximate "instrument response function". The raw spectra are then divided
-
0
2204
F R E o u E N C Y ( cm- I ) Figure 3. A comparison of the resolving capabilities of the CVF and FTIR techniques. (a) CVF spectrum of CO emission obtained from earlier work in this laboratory (ref 16), with a resolution of -30 em-'. (b) A time-resolved FTIR spectrum obtained in the present experiment with 10 Pa of acetone and -400 Pa of Ar flowing through the cell and a time delay after the exciting laser pulse of -30 ws. The Ar serves to collisionally relax the CO rotational distribution while preserving the nascent vibrational distribution. The resolution is -0.25 em-'.
0.I
0
I
3
2
4
i
V
Figure 4. Results of the vibrational analysis of the CO emission spectrum. (a) A Boltzmann plot of the three observed u levels. The points describe a vibrational temperature of -2030 K. (b) Relative populations observed in 'u = 1-3 ( 0 ) plotted with the calculated populations at -2030 K (0).
by this instrument response function to obtain a spectrum that reflects the true emission intensities. 111. Results A. 'VibrationalDistribution. The timeresolved FTIR technique
is unique because it allows high-resolution spectra of emitting molecules to be obtained over broad wavelength ranges in a relatively short period of time. Such high-resolution spectra can be analyzed directly to obtain rovibrational state populations, without the deconvolutions necessary in lower resolution work or the complications of upper and lower state populations in laser gain/absorption methods. Figure 3a shows the spectrum of CO obtained from an earlier low-resolution study in this laboratoryI6 of 193-nm acetone fragmentation. In that work, a circular variable filter (CVF) was used to resolve the CO emission, with an ultimate resolution of -30 crn-'. Such spectra were used to determine the distribution of vibrational states in the CO by first relaxing the rotational distribution to 300 K with 670-1330 Pa of Ar buffer and then varying the vibrational temperature to obtain the best fit to the data. The result of that work was a vibrational temperature of 1200 K. Figure 3b shows a spectrum obtained in the current work with 10 Pa of acetone and -400 Pa of Ar flowing through the cell and a time delay after the exciting laser pulse of -30 ps. The resolution is -0.25 cm-I, and the figure dramatically illustrates the increased information content inherent in the interferometric technique. Under the given pressure and time delay conditions, CO will readily relax rotationally via collisions with Ar but will retain its nascent vibrational distribution. Thus, we simply measure the areas of the peaks, divide by the appropriate Einstein spontaneous emission coefficients, and sum over the vibrational states to determine the relative vibrational populations. Only lines that are fully resolved in all three u levels are used for such an analysis. They are R(17), R(12), R(10), R(9), R(5), R(1), R(O), P(5), P(6), P(8), and P(11).33
-
Woodbridge et al.
5390 The Journal of Physical Chemistry, Vol. 92, No. 19, 1988 -4
2 7 Po, 6 p s c c DELAY I IO COLLISIONS
-7
I-
e
I I I I ( ( I / I I I I I I
m
0
k
1000
2000
J ' ( J ' + I)
2
3
0 18
m
z
e
cc
E
a
EXPT
0 3360 K
u
J
>
= l
-
I-
a 0 a
m
2 W I-
010
W
5
z
c a
-I W [L
IO 0 Po, 30 p s c c D E L A Y
0
3 t o r r Ar 5 600 COLLISIONS ( A r )
, I900
.
2000
J Figure 6. Rotational-state analysis for the u' = 1 level of CO. (a) A
Boltzmann plot incorporating only the fully resolved lines, indicating a temperature for rotation of -3360 K. (b) Relative populations observed ( 0 )and calculated (0)plotted vs J .
.. 2100
F R E O U E N C Y (ern-')
Figure 5. P branches of the CO emission for several different pressures
and time delays. See text for details. The results of such an analysis, a vibrational Boltzmann plot and an accompanying population plot, are shown in Figure 4. The fit is quite good, and, within the 95% confidence limit, indicates K. that CO is born with a vibrational temperature of -2030:# The Einstein coefficients used throughout the experiment were determined by computing the matrix element for each transition using the polynomial fit of the electric dipole moment function developed by Chackerian and Tipping.34 It should be noted that within the S/N limits of the present work we see no evidence for emission from u' 2 4. B. Rotational Distribution. The goal of the present work is to study the energy disposal in the photofragments by observing rotationally resolved emission spectra under collision-free conditions. The slow detection time constant of -4-6 I.IS in the present system requires that we stay below ---Pa total pressure if we are to meet this objective of observing products before collisions occur. Such low pressures, coupled with the fact that -78% of the CO products are born in u = 0, mean that the signal levels are very low. Therefore, an appreciable number of scans must be coadded to give an adequate S/N ratio for spectral analysis. We are limited in this respect by the lifetime of the laser hardware, -10' pulses or -8.8 h of continuous operation a t the nominal repetition rate of -316 Hz.'O It is necessary, therefore, to strike a balance between the length of each mirror scan, which determines the ultimate resolution, and the number of scans, which determines the signal-tenoise ratio. At present, under low-pressure conditions, we are limited to 1-cm-' resolution and 150 scans per spectrum, which requires -4.5 X lo6 laser shots. Thus, under ideal conditions for acetone photolysis we can acquire two spectra of CO under collision-free conditions before laser service is required.
-
-
(33) Molecular constants for the calculation of line positions taken from:
Huber, K. P.; Herzberg, G. Consranrs ofDiatomic Molecules; Van Nostrand
Reinhold: New York, 1979; p 166. (34) Chackerian, Jr., C.; Tipping, R.H. J. Mol. Spectrosc. 1983, 99, 431.
Figure 5 shows the emission spectrum of CO between 1900 and 2150 cm-' recorded at several different pressures and time delays. This region encompasses the u' = 1 P branch as well as emission from the u' = 2 and 3 P and R branches. Spectra a and b were obtained with -1.3 and -2.7 Pa, respectively, of acetone flowing through the chamber, and a time delay of -6 ,us after the laser pulse. Under these conditions the CO will undergo -0.5 and 1 gas kinetic collision, respectively, before we observe the IR emission. We note the apparent relaxation in (b) from the more highly excited distribution in (a) as evidence of efficient rotational relaxation of CO by the acetone parent. The spectrum in Figure 5c was obtained with -2.7 Pa of acetone flowing through the chamber and a time delay of -20 ,us, corresponding to -3 gas kinetic collisions. Substantial relaxation is evident. Figure 5d is the high-resolution, high-pressure spectrum shown in Figure 3b that was used for the vibrational-state determination, retransformed here to match the 1-an-'resolution of the low-pressure spectra. The rotational-state populations measured from this spectrum can be described very accurately by a Boltzmann distribution, with a rotational temperature of -480 K. The low 1-cm-' resolution, the mixture of overlapped vibrational bands, and the poor S/N of the low-pressure spectra make it nearly impossible to obtain full resolution of the lines from each individual vibrational state. Many of the peaks in the spectra are overlapped, containing contributions from two or three excited vibrational levels. The first excited level, u' = 1, does present some peaks that are uncontaminated by contributions from higher lying vibrational levels, and these peaks are used to obtain the Boltvnann plot shown in Figure 6a. This plot and the accompanying population plot shown in Figure 6b describe our lowest pressure data, 10.5gas kinetic collision. The plots suggest that CO is born with a highly excited, approximately Boltzmann distribution of rotational states that can be described by a temperature of 336O$'Am K in the 95% confidence limit. However, we suspect that even the datum with the fewest number of collisions does not describe the nascent distribution, since data obtained at -0.8-1.0 collision, Figure 5b, already show evidence of rotational relaxation. It is our belief that efficient rotational relaxation of CO by the parent acetone degrades the nascent distribution even after S0.5 gas kinetic collision. Thus, the nascent rotational distribution may be even hotter than Figure 6 indicates.
-
-
Photofragmentation of Acetone at 193 nm 42
41
40
39
38
37
36 35
The Journal of Physical Chemistry, Vol. 92, No. 19, 1988 5391
J' V/'
700
3
- -m-(6191 -419
2
600 -
v/= I
500 -
v"
-I -w
- - - - - --
400--
0
E
2
300-
200
5
-
I O COLLISIONS
loo-
bl
0-
/CH3COCH3
Figure 8. Schematic energy level diagram for the molecular species of
5
3.0 COLLISIONS C)
5 600
COLLISIONS ( A r )
interest in acetone photolysis. triplet state, T I , and the rate of internal conversion to high-lying vibrational states on the ground-state surface are both approximately 2 orders of magnitude faster than the radiative decay rate. Some of the first studies of the fragmentation dynamics of acetone in this wavelength range were performed by Hancock and Wilson20using a molecular beam time-of-flight (TOF) apparatus. Exciting acetone at 266 nm, they observed CH3 and CH3C0 as major products, but no CO. Their observed flight times for the two radical products indicated that -43% of the 135 kJ mol-' available energy appeared as translation while the remaining --57%, or -77 kJ mol-', was partitioned into internal modes of the fragments. The absence of any signal from C O is a clear indication that at 266 nm the internal excitation of the C H 3 C 0 fragment is insufficient to overcome any barrier to further decomposition. This is in agreement with work by Watkins and Word,35 who measured a barrier for C-C bond breaking in C H 3 C 0 of -72 kJ mol-'. Baird and K a t h ~ a calculated l~~ the equilibrium geometries and energetics of the low-lying states of CH3C0, and their work also predicts a large barrier for production of CH3 and CO. Figure 8 shows the relevant energy levels along the assumed reaction coordinate. In addition to identifying the primary photodissociation products, Hancock and Wilson also measured the angular distribution of the fragments and found it to be isotropic. This is consistent with the measurements of the nonradiative rates made by Lee, whose data indicate that any photochemistry initiated from the first excited singlet state will proceed through an intermediate with a lifetime much longer than the vibrational or rotational periods of the acetone molecule. Analogues of acetone have also been studied by molecular beam TOF. Kroger and Riley fragmented CH3CO13' and CF3CO138 in a beam at 266 nm and observed the products by TOF mass spectroscopy. CH3COI dissociates to give three products: I atoms, CH3, and CO. The anisotropic angular distribution of the iodine atoms indicates that the directly dissociative excited state of the acetyl iodide accessed at 266 nm has a lifetime of 51.4 X s. The angular distributions of the CH3 and CO fragments, however, indicate that the remaining acetyl radical has a lifetime of several rotational periods, roughly lo-'' s. Kroger and Riley's analysis suggests that a steep repulsive potential along the C-I bond is accessed by the 266-nm photon, which results in a rapid fragmentation into C H 3 C 0 and I. This deposits -80% of the available excess energy into internal modes
1 dl
I895
1915
1935
FREQUENCY ( c m - l )
Figure 7. The CO emission spectrum around 1900 cm-I. These are expansions of the spectra shown in Figure 5 and illustrate the excitation observed in the upper J states of u' = 1-3.
The upper vibrational levels, u' = 2 and 3, do not present enough fully resolved peaks to allow a meaningful analysis. Figure 7 shows the region of the spectrum around 1900 cm-', where we still see significant contributions to the spectral intensity from u' = 1, as well as overlapped lines from v' = 2 and 3. The intensity in this region suggests that LJ' = 2 and 3 are born with rotational excitation comparable to that in u' = 1, which exhibits significant rotational excitation in levels as high as J 2 50.
IV. Discussion From several thorough studies'+'* of photoprocesses in acetone at wavelengths of 200 nm). These two wavelength regions access distinctly different electronic states that give different fragmentation products. A . X > 200 nm. Breuer and Lee'9 and Hansen and Lee22 thoroughly studied the rates for intersystem crossing, internal conversion, and radiative decay of the excited '(n,a*) state when accessed between 260 and 3 13 nm. They conclude that the acetone radiative lifetime is 1.6-2.7 ps over this wavelength range, much longer than the rotational period of the molecule. In addition, they note that the rate of intersystem crossing to the first excited
-
-
(35) Watkins, K. W.; Word, W. M. Int. J. Chem. Kinet. 1974, 6, 855. (36) (a) Baird, N. C.; Kathpal, H.B. Can. J . Chem. 1977, 55, 863. (b) Baird, N. C. Pure Appl. Chem. 1977, 49, 223. (37) Kroger, P. M.; Riley, S. J. J . Chem. Phys. 1977, 67, 4483. (38) Kroger, P.M.; Riley, S. J. J. Chem. Phys. 1979, 70, 3863.
5392 The Journal of Physical Chemistry, Vol. 92, No. 19, 1988 of CH3C0. The excited CH,CO, after living for a few rotational periods, undergoes a unimolecular decomposition to CH3and CO, depositing -70% of its available excess energy into internal modes of these fragments. The energy deposition in the CH3 and C O fragments are modeled successfully by statistical theories of unimolecular decay for a long-lived activated complex. In contrast to CH3COI, CF3COI appears to fragment by a nearly instantaneous three-body process on a time scale of s. The iodine atom angular distribution and TOF data indicate a very similar C-I interaction to that in acetyl iodide, but the CF3C0 fragment does not persist as such, undergoing immediate fragmentation to CF, and CO. The precise change that fluorine substitution makes to the potential surfaces governing the reaction remains unclear. B. X < 200 nm. Potzinger and Von BiinauI7 photodissociated acetone at 185 nm in a static cell and observed the final products, CO and C2H6, by mass spectroscopy. These products are the result of the mechanism of eq 1, with rapid recombination of the methyl radicals in the static cell to produce ethane. Soloman et a1.I8 fragmented acetone with a broad-band Hg arc and used the photolysis mapping technique (reacting methyl radicals with metals deposited on the inside of the reaction vessel) to detect the angular distribution of the products. In contrast to the results of Hancock and Wilson at 266 nm, they observed an anisotropic distribution of photofragments. Unfortunately, the broad waveiength range utilized for the photolysis did not allow the authors to draw any conclusions about what excited states were the precursors of the observed products. One might surmise, however, that the dominant excitation was in the vacuum-UV, where the absorption cross section of acetone is very large. Therefore, the excited states accessed at these shorter wavelengths may have much shorter lifetimes than the '(n,r*) state accessed in the near-UV region. More recent work on acetone fragmentation comes from the investigation of Baba et al.,15 who studied the multiphoton ionization (MPI) of acetone at 248 and 193 nm. At 193 nm they observed strong ion signals from CH3COCH3+and CH3CO+, as well as a signal from CO+. The laser power dependences of the ion signals suggest that the first step in the acetone dissociation at 193 nm is a rapid fragmentation to yield CH3 and a transient CH3CO. Very recently, vacuum-UV absorption spectra of acetone monomers and clusters cooled in a supersonic jet have been obtained.26-27These spectra are thought to illuminate the subtle details of the vibronic coupling that links the ground state with the '(n,3s) Rydberg state accessed at 193 nm. The interpretation is that when molecules cluster, the van der Waals interactions perturb the potential surfaces of the various states just enough so that vibrational modes which were strongly predissociative in the monomer are no longer as strongly coupled to the dissociative surface. These modes then show up as identifiable peaks in the cluster spectrum. Two such bands appear in the cluster spectrum of acetone, and they have been identified as the vg C-C-0 bend and the VI6 out-of-plane deformation mode. These modes appear to be strongly coupled to a mixed (S,,T,) dissociative surface which is thought to correlate in the asymptotic limit to CH3C0 and CH3, consistent again with a two-step picture of acetone photofragmentation. C. Energy Disposal. If one accepts the concept of a two-step bond breaking in the fragmentation of acetone, then the possible patterns of excitations in the fragments can be described. The departure of the first methyl radical would leave the C H 3 C 0 radical with some excitation in the C-C-0 bend, in addition to that provided by the vibronic coupling of the So and S2 surfaces. Indeed, in both CH3COI and CF3COI Kroger and Riley observed approximately 80% of the available excess energy being partitioned into internal states of the CX3C0fragment, much of it presumably in the C-C-O bend. Such excitations can have dramatic effects on the subsequent rotational distributions of the departing fragments, as Band et al.39and Morse and Freedi4 have shown for
-
(39) Band, Y . 3.;Freed, K. F.; Kouri, D. J. J . Chem. Phys. 1981, 74,4380.
Woodbridge et al. linear triatomics such as H C N and ICN. Although it is not possible in this experiment on the CO fragment to say anything definitive about the initial step of the fragmentation, we can say something about the unimolecular decomposition of the energized C H 3 C 0 radical. From heats of formation at 298 Km we know that approximately 278 kJ mol-' remains to be partitioned into internal and translational modes of the CH3 and CH,CO fragments after the first bond-breaking step in acetone fragmentation at 193.3 nm. We assume that the partitioning of this energy falls somewhere between the patterns observed by Hancock and Wilson at 266 nm and those observed by Kroger and Riley for CX3COI, also at 266 nm. For our purposes, assume that -30% of the energy will be partitioned into translational and -70% into internal degrees of freedom of the two fragments. The C H 3 C 0 and CH3 fragments will then have 195 kJ mol-' partitioned between their respective internal modes. It has been inferred from previous analysis of the CH, radical emission in our own laboratoryI6 that -35 kJ mol-' is carried in internal modes of at least one CH3fragment. Assuming that the maximum energy resides in the first methyl radical dissociated, then at least 160 kJ mol-' is left in the energized CH,CO radical. The endoergicity of the reaction CH3CO CH3 CO (4)
-
-
-
+
is -57 kJ mol-', with an expected barrier of -72 kJ mol-'.35 With these available energies and barrier heights we can attempt to model the internal-state distribution of the CO fragment using the "pure impulsive" approach of Busch and Wilson.41 As its name implies, the pure impulsive model treats the dissociation as an abrupt separation with the available energy being partitioned by conservation of linear momentum into the recoil of the two carbon atoms. The CO carbon atom is, of course, constrained by the oxygen atom, and the initial linear momentum imparted to the carbon atom is distributed by conservation laws into the vibrational and rotational modes of the CO molecule. We assume that internal excitation in the methyl radical is negligible, since the light hydrogen atoms move with the heavier carbon atom during the impulsive recoil. This is, of course, contrary to an RRKM picture of dissociation, in which the CH, would be excited by simple randomization of the available energy. If, however, the time scale for CH3C0 breakup is very short compared to the time required for free flow of energy in the molecule, such randomization may not occur. The partitioning of the available energy into translation of the two fragments and the vibrational and rotational modes of the CO molecule is then given by
E, = (1 - M/Mm)Eavl sinZx
(7)
where x is the C-C-0 bond angle, p is the reduced mass of the two carbon atoms, M,,, is the reduced mass of the CH3 and CO fragments, and Ea,.,is the excess energy available to the fragments, -103 kJ mol-'. Based on the equilibrium bond angle of 128' calculated by Baird and K a t h ~ a l -15 , ~ ~ kJ mol-' would be partitioned into vibration of the CO molecule and -25 kJ mol-' would be partitioned into rotation. This is in approximate agreement with the observed distributions of 17 kJ mol-] in vibration and -28 kJ mol-' in rotation, obtained by calculating RT at the observed temperatures. Alternatively, given the observed distributions the bond angle and available energy can be determined by a simple rearrangement of eq 5-7. The resulting bond angle is 127.8O and the available energy is 116.0 kJ mol-', which is quite consistent with our conjectures about the energy partitioning in the initial stages of the fragmentation and the calculated geometry of ground-state CH3C0.
-
-
-
(40) (a) Chase, M. W., Jr.; Davies, C. A,; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N., Eds. J . Phys. Chem. Re/. Data 1985, 24, Suppl. No. 1. (b) Chase, M. W., Jr.; McDonald, R. A.; Syverud, A. N.; Curnutt, J. L., Eds. J . Phys. Chem. Ref.Data 1977, 6, Suppl. No. 1 . (41) Busch, G. E.; Wilson, K. R. J . Chem. Phys. 1972, 56, 3626.
J. Phys. Chem. 1988, 92, 5393-5397 V. Conclusions The nascent vibrational and near-nascent rotational-state distributions of the CO fragment observed following the 193-nm photofragmentation of acetone are consistent with a proposed two-step mechanism of bond breaking in the fragmentation. The vibrational distribution is mildly excited and can be fit by a temperature of -2030 K. The rotational distribution is more highly excited, exhibiting population out to J = 45-50, and can be described approximately by a temperature of -3360 K. The distributions can be modeled successfully by a purely impulsive mechanism for breaking the C-C bond of the bent acetyl radical. The implication is that the rovibrational modes of the CO molecule are coupled to the dynamics of the dissociation by the simple conservation of linear momentum that results from the recoil of the two carbon atoms. We await data from TOF or Doppler spectroscopy studies on the vacuum-UV fragmentation of acetone to give additional quantitative results for energy disposal in the initial step of this fragmentation. At present we are working on a higher speed
5393
time-resolved FTIR system that will allow us to obtain spectra at higher resolution over a broader wavelength range than is currently feasible. With this we hope to obtain unambiguous internal-state distributions for the CH3 fragments from the photodissociation of acetone and discern any bimodalities that might result from two-step bond breaking.
Acknowledgment. We are grateful to the National Science Foundation and the National Bureau of Standards for the funds to obtain the FTIR and to the Department of Energy for the laser and the support of this research. We also thank Dr. D. J. Donaldson for many helpful discussions. We also acknowledge the memory of E. K. C. Lee, with whom E.L.W. did undergraduate research in 1984-1985. Prof. Lee fostered excellence and enthusiasm in his students and served as an inspiration to a younger generation of scientists who continue to pursue research in photochemistry with the vigor and sense of purpose that exemplified his work. Registry No. (CH&CO, 67-64-1; CO, 630-08-0.
Single-Vibronlc-Level and Excitation-Energy Dependence of Radiative and Nonradiative Transitions In Jet-Cooled S, Pyridine EIiel Villa, Division of Chemistry, Naval Research Laboratory, Washington, D.C. 20375-5000
Aviv Amirav, School of Chemistry, Tel-Aviv University, Ramat Aviv 69978, Tel Aviv, Israel
and Edward C. Lim* Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received: December 28, 1987)
Optical excitation of the combination bands containing one quantum of out-of-plane vibrations of az symmetry leads to relatively large decreases in the quantum yield of fluorescence and quantum yield of triplet formation in jet-cooled S1,pyridine. This observation as well as the dramatic, and mode-independent, decreases in the quantum yields at higher excitation energies are attributed to the IVR (intramolecular vibrational redistribution) induced internal conversion to the ground electronic state. It is proposed that the IVR, which is mode selective at lower excess vibrational energies, leads to population of the exceptionally good promoting and/or accepting mode(s) for the internal conversion process. Possible candidates for this “magic mode(s)” are briefly discussed.
Introduction Electronicallay excited pyridine provides considerable challenges to spectroscopists interested in fundamental understanding of electronic relaxation processes. The most notable feature of the photophysics of SI(lowest excited singlet) pyridine has been the occurrence of efficient internal conversion to the ground state (So), which effectively competes with rapid intersystem crossing to the In a static gas sample of pyridine, the internal triplet conversion from the zero-point vibrational level has been deduced3 to take place with a quantum yield of -0.7 and rate constant of 1Olos-l. This is about 2 orders of magnitude greater than the corresponding rate in pyrimidine and p y r a ~ i n e . An ~ explanation that has been proposed for the very rapid internal conversion as So intersystem crossing well as for the rapid T1 (lowest triplet)
-
-
(1) Knight, A. E. W.; Parmenter, C. S. J. Chem. Phys. 1976, 15, 85. (2) Lemaire, J. J. Phys. Chem. 1967, 71, 612. (3) Yamazaki, I.; Murao, T.; Yoshihara, K.; Fujita, M.; Sushida, K.; Baba, H. Chem. Phys. Lett. 1982, 92, 421. (4) Yamazaki, I.; Fujita, M.; Baba, H. Chem. Phys. 1981, 57, 431.
0022-3654/88/2092-5393$01.50/0
of pyridine is that vibronic perturbation of the lowest n r * state (singlet and triplet) by higher lying UT* states (singlet and triplet) leads to substantial distortion (frequency change) and displacement (geometry change) along the potential energy curves of the vibronically active out-of-plane bending modes (pseudo Jahn-Teller effect5s6),which greately increases the Franck-Condon factors for the S1 So internal conversion and T1 So intersystem crossing (proximity effect).’-” Since the second excited singlet
-
-
( 5 ) Moffit, W.; Liehr, A. D. Phys. Rev., 1957,106, 1195; Liehr, A. D. 2. Naturforsch. 1961, 16, 641. (6) Hochstrasser, R. M.; Marzzacco, C. A. In Molecular Luminescence; Lim, E. C., Ed.; Benjamin: New York, 1969; p 631. ( 7 ) Lim, E. C. In ExcitedStates; Lim, E. C., Ed.; Academic: New York, 1977; Vol. 3, p 305, and references therein. (8) Lim, E. C. J. Phys. Chem. 1986,90, 6770. (9) Sushida, K.; Fujita, M.; Takemura, T.; Baba, H. Chem. Phys. 1984, 88,221. (10) Selco, J. I.; Holt, P. L.; Weisman, B. .IChem. . Phys. 1983, 79, 3269. (1 1) Buma, W. J.; Groenen, E. J. J.; Schmidt, J. Chem. Phys. Lett. 1986,
127, 189.
0 1988 American Chemical Society
